Locking mechanisms have been used for decades to lock opposing rams of a blowout preventer (BOP) closed. Those familiar with BOP operations recognize the safety advantages of automatically locking the BOP rams closed, ensuring that the BOP rams remain closed even if hydraulic pressure to the BOP is lost or removed, and only allowing the BOP rams to open when hydraulic pressure is intentionally applied to the BOP for the purpose of opening the BOP rams.
Early versions of BOP locking devices included complex locking assemblies with numerous components and biasing springs. One early version of a blowout preventer locking mechanism is disclosed in U.S. Pat. No. 3,242,826. Other locking mechanisms for ram-type blowout preventers are only capable of locking each ram shaft closed in a single locking position. Although various sizes of ram sealing blocks and ram shearing blocks are used for different size oilfield tubulars, BOP locking devices which lock in a single position, such as the devices disclosed in U.S. Pat. Nos. 4,188,860 and 4,519,571, have questionable reliability under many applications. The nominal diameter of the oilfield tubular may vary slightly as a function of tubular manufacturer. The exterior surface of the tubular may have experienced wear or erosion, thereby reducing the outer diameter of the tubular for engagement with the ram sealing block. Packer seal wear on the sealing block head requires proper axial spacing of the ram shaft position to ensure that an effective seal will be made with the oilfield tubular passing through the BOP. Seal wear on the opposing shearing ram blocks also requires different axially spaced ram locking positions to ensure Chat the mating shear rams will seal off high pressure in the wellbore below the rams of the BOP. Using a single locking position actuating mechanism, field adjustments need to be repeatedly made by a trained operator to ensure that the locked position of the ram shaft will provide the desired sealing operation when the BOP was closed.
Other BOP locking mechanisms have included locking dogs or stops which are external to the blowout preventer ram cylinder. An external BOP locking device, as disclosed in U.S. Pat. No. 3,918,478, is no longer widely used. Other BOP ram locking devices include tail rod extensions which move within a locking housing in a direction generally perpendicular to the axis of the ram and the ram housing. U.S. Pat. Nos. 4,601,232 and 4,969,627 each disclose locking mechanisms with a lock housing separate from the ram housing. The added cost and complexity of these designs has reduced their acceptance for many applications.
In recent years, ram locks for blowout preventers which permit locking of the ram shaft in multiple positions have included axially spaced teeth along the ram or along another member which is axially centered with respect to the ram axis. U.S. Pat. No. 4,290,577 discloses a ram lock with multiple locking positions formed by the threads on the surface of a rod. These threaded rod surfaces cooperate with a unidirectional clutch assembly to lock the ram closed in any one of multiple positions. Torque rotates a lock nut, thereby allowing for ram locking in multiple positions. The locking mechanism is automatically set each time the ram is closed, and the locking system maintains the ram closed after hydraulic closing pressure is released. On the application of an opening pressure which releases the clutch mechanism, the locking mechanism opens and allows the ram to open. The mechanism as disclosed in this patent is also complex, and repair and maintenance costs are high.
U.S. Pat. No. 4,840,346 discloses a locking mechanisms which includes axially spaced teeth on the exterior of a central ratchet cylinder. A plurality of flexible finger members are circumferentially spaced about the ratchet cylinder, and are axially fixed with respect to an elongate rod which moves the sealing head into engagement with the oilfield tubular. U.S. Pat. No. 3,941,141 also discloses a locking assembly for locking a ram of a blowout preventer. The central rod includes axially spaced teeth, and a plurality of slip members are brought into locking engagement with the central rod by cooperation between the outer surface on the slip members and the inner surface on a wedge member.
U.S. Pat. No. 5,025,708 discloses a ram locking mechanism which is capable of locking in multiple positions. The assembly includes a plurality of circumferentially spaced locking rods, each having locking teeth for engagement with a respective locking segment. The locking segments are spring biased for engagement with the locking teeth, and may be moved out of engagement with the locking rods by hydraulically controlled movement of an unlocking piston with tapered unlocking fingers. BOP ram locking devices which rely on axially spaced teeth are capable of locking in multiple positions, although the number of locking positions is determined by the axial spacing of the locking teeth. As the teeth are spaced axially closer together, machining costs increase and the number of teeth required to reliably withstand the ram opening pressure increases. The '708 patent minimizes this problem by allowing the teeth on one locking rod to be axially offset with respect to the teeth on another locking rod.
U.S. Pat. No. 4,305,565 discloses a ram lock for a blowout preventer which includes a plurality of wedge members for sandwiching between a tapered interior conical surface on the ram housing and tapered exterior conical surface on a locking cone movable by a piston. This type of locking device also includes numerous components. While the locking device is capable of locking in multiple positions, the varying diameter of the frustoconical surfaces on both the ram housing and the locking cone only ensures that there will be line contact rather than planar surface contact between the wedge member and both the ram housing and the locking cone. When high well pressures are attempting to force the ram to an open position, these forces are concentrated along lines of contact rather than mating planes of contact, thereby creating high stresses and excessive wear between components.
The disadvantages of the prior art are overcome by the present invention, and an improved blowout preventer with wedge ram locks is hereinafter disclosed. The locking mechanism of the blowout preventer as disclosed herein is relatively simple yet rugged, and is highly reliable for locking a ram in an infinite number of locking positions within a broad locking range.